In order to widen the family of characteristics for radial compressor stages, stabilizers are used in the induction area of the compressor wheel in a multiplicity of the latest generations of radial compressor stages.
The market demand for ever higher pressure ratios in compressors of exhaust-gas turbochargers is never-ending. However, the process of increasing the pressure ratio by increasing the rotation speed without changing the compressor stage design is subject to limits, since the surge limit and choke limit, which limit the useful range of characteristics, converge as the rotation speed increases. The useful range of characteristics therefore decreases continuously in the direction of higher pressure ratios. In order to counteract this and to keep the useful range of characteristics as broad as possible even at high pressure ratios, it is possible to use a diffusor with a smaller flow cross section, while the compressor wheel design remains the same and the compressor wheel size is not changed. The surge limit is thus shifted in the direction of lower volume flows, resulting in a wider useful range of characteristics without changing the wheel choke limit. One disadvantage in this case is that the efficiency is reduced, particularly when on partial load. This disadvantage can be avoided by using appropriate measures to increase the stability of the given compressor stages at maximum load. This can be achieved by blowing air in, on the housing side, into the flow channel in the intermediate area, where there are no blades, between the rotor blades of the compressor wheel and the guide vanes of the diffusor. The dynamic stability in the region of high pressure ratios can be increased by blowing in air.
Another possible way to increase the pressure ratio and to avoid convergence of the surge limit and choke limit is adaptation of the compressor wheel design. The stability and therefore the useful range of characteristics can be achieved by increasing the “backsweep” of the compressor wheel. The “backsweep” denotes the angle at the compressor wheel outlet between a blade with a radial trailing edge and one with an outlet angle which is positioned at a flatter angle in the tangential direction, in the opposite direction to the wheel rotation direction. The increase in the “backsweep” results in the need to increase the wheel circumferential speed in order to achieve the same pressure ratio. It is therefore necessary to increase the rotation speed more than proportionally in order to achieve a higher pressure ratio. However, this is limited by the compressor wheel material limits, or a change must be made to a material with better mechanical characteristics. Materials such as these are considerably more expansive. In comparison to this solution, the process of blowing air in has cost advantages, since an existing compressor stage is suitable for achieving higher pressure ratios, and there is no need for a costly change in the material of the compressor wheel.
“Centrifugal Compressor Flow Range Extension using Diffusor Flow Control”, (Gary J. Skoch; Army Research Laboratory, Vehicle Technology Directorate, Cleveland, Ohio; Dec. 5, 2000) discloses a radial compressor with a downstream diffusor, in which compressed air is blown in the flow direction into the flow channel between the compressor wheel and the diffusor, using the Coanda effect nozzles.
In the Coanda effect (described in U.S. Pat. No. 2,052,869) is a flow effect on the basis of which a rapidly flowing fluid (gas or liquid) which is flowing along a surface of a solid body adheres to the surface of this body and is not separated from the surface.
The compressed air nozzles are arranged in the housing wall which bounds the flow channel, and are firmly screwed to the compressor housing. They can move within the openings, so that the induction direction can be varied. The nozzles are connected via a pipeline to an external compressed-air supply.
CH 204 331 discloses a device for preventing jet separation in compressors. In this case, parts of the flow are sucked away through extraction openings in the area of the guide wheels, and are then fed back into the flow again, further upstream. In this case, the flow is reintroduced by means of circumferential slots, in the form of nozzles, aligned in the flow direction.